The use of a capacitance probe to measure liquid levels has been disclosed in the prior art. For instance, in U.S. Pat. Nos. 3,958,159 and 3,864,974 (Rauchwerger), a capacitance probe including a tensioned insulated wire is used to measure changes in the liquid level of a container in cooperation with an electrical circuit using active elements. Another capacitance sensor using a tightly stretched insulated wire disposed inside of a closed container is described in U.S. Pat. No. 4,025,846 (Franz et al). Other examples of capacitance probes are described in U.S. Pat. No. 2,774,959 (Edelman et al) and U.S. Pat. No. 3,878,719 (Luttmann et al).
In addition, the circuit disclosed in the Rauchwerger patents referred to above, circuits which are used with a capacitance probe are disclosed in other patents. For instance, in U.S. Pat. No. 3,918,306 (Maltby), an impedance sensing network is used to measure transmission fluid level. A resistance-capacitance network is disclosed in U.S. Pat. No. 4,010,650 (Piatkowski, Jr.) for determining the liquid level in a reservoir. Another measuring apparatus which utilizes a capacitance transducer and associated circuit is disclosed in U.S. Pat. No. 4,086,528 (Walton).